1. Technical Field
The present invention relates to a process for electroplating metals, and particularly to an electroplating process in which a build-up of metal occurs in the electroplating bath. The build-up can be due to (i) a current efficiency of the electroplating cell anode greater than the current efficiency of the electroplating cell cathode, and (ii) chemical dissolution or corrosion of the electroplating cell anode in the plating solution.
2. Description of the Prior Art
U.S. Pat. No. 4,906,340, issued Mar. 6, 1990, discloses a process for electroplating metals. The process employs an electroplating cell comprising a bath containing a plating solution of a metallic salt, a cathode comprising a workpiece to be plated, and a soluble anode. The anode current efficiency of the cell is greater than the cathode current efficiency. This causes more metal to dissolve from the anode than is plated at the cathode, in turn causing a build-up of dissolved metal in the electroplating cell. The current that is not used for plating at the cathode generates hydrogen in the cell.
The process includes the steps of providing an electrowinning cell which includes at least one insoluble anode, at least one insoluble cathode, and a bath which communicates with the bath of the electroplating cell. The plating solution is circulated between the electroplating cell and the electrowinning cell.
A source of direct current is connected across the anode and cathode of the electrowinning cell so as to cause depletion of the metal from the plating solution onto the electrowinning cell cathode. The amount of current in the electrowinning cell is controlled to be at least substantially equal to the amount of current flowing through the electroplating cell which results in the generation of hydrogen.
The patent discloses the following equation for calculating the amount of current I.sub.w (amp.) to use in the electrowinning cell: ##EQU1## wherein
E.sub.p =cathode efficiency in electroplating cell (%)
E.sub.w =cathode efficiency in electrowinning cell (%)
I.sub.p =current in electroplating cell (amp.)
In certain electroplating processes, for instance the electrogalvanizing process, it has been found that the apparent anode current efficiency in the electroplating cell is much greater than 100% due to significant chemical dissolution or corrosion of the anode in addition to electrochemical dissolution of the anode. This means that in such processes, if the electrowinning cell is operated at a current at least substantially equal to the current flowing through the electroplating cell which results in the generation of hydrogen, the concentration of the metal in the plating solution will continue to rise due to the chemical dissolution. In the case of electrogalvanizing, by way of example, this build-up due to chemical dissolution can be substantial.
The patent is also based on the assumption that the current efficiency of the anode is 100%, which is why the value "1" appears in the above equation (1). In plating with zinc, for instance in electrogalvanizing, the anode efficiency in the electroplating cell is at least close to 100%, and the assumption is reasonable. However, when plating with other metals, such as nickel, when the chloride ion concentration is too low, or other soluble anodes when the anode current density is too high, the anode efficiency in the electroplating cell may be less than 100%. Thus, application of the above equation to an electrowinning process, where the anode in the electroplating cell is a metal having an efficiency (E.sub.p) less than 100%, will give too high a value for I.sub.w. This in turn will result in a depletion of metal in the electroplating bath (assuming no dissolution of the electroplating anode).